The knocking noise is audible on passenger footage captured aboard the plane. The pilot turned the plane around after hearing the banging.

"The captain immediately returned to Seattle, declaring an emergency for priority landing. The aircraft was in the air for 14 minutes," the airline said. "After landing, the ramp agent was found inside the front cargo hold, which is pressurized and temperature controlled. The ramp agent appeared OK, and was transported to the hospital as a precaution. We are actively investigating the matter."

In a blog on its website, the airline said the agent was an employee of Menzies Aviation and had passed a drug test this afternoon.

"Upon exiting, he told authorities he had fallen asleep," the airline said.

"The employee started work at 5 a.m. and was scheduled to end his shift at 2:30 p.m. During a pre-departure huddle, the team lead noticed the employee was missing. The team lead called into the cargo hold for the employee and called and texted the employee’s cell phone, but did not receive an answer. His co-workers believed he finished his shift and went home.

"All ramp employees have security badges. They undergo full criminal background checks and drug screening prior to being hired. They are also subjected to random drug tests throughout their employment. There were 170 passengers and six crew members on Flight 448. The aircraft landed in Los Angeles at 6:17 p.m."

The crew member was treated and released at Highline Medical Center in Burien, Washington, hospital spokesman Scott Thompson said.